Altogether, about 5% of starch digestion happens in the mouth. However, it will vary a bit depending on how long you chew, how much moisture there is in the mouth, and how much moisture the starch can absorb.
The digestion of starch occurs due to the combined effects of chewing (mastication) and the enzyme, salivary amylase. This enzyme is produced by your salivary glands. It coats the food particles in your mouth, and breaks the bonds that link together the monomeric sugar units that make up a starch molecule. This chemical breakdown process is called starch hydrolysis; its end products are smaller sugar chains, including maltose, maltotriose, and dextrins. When you chew on a piece of whole grain bread, and find it becoming sweeter as you continue to chew, it's because you are converting starch into these sugars.
Hydrolysis depends on the presence of water, so food particles that absorb more water may get digested faster. Moreover, the more you chew the more saliva you produce. Thus, chewing is responsible for more than mechanical breakdown -- it also triggers the release of more amylase and water.
In one experiment (see last link below), researchers asked volunteers to chew mouthfuls of either bread or spaghetti. Then the researchers compared how much starch digestion had occurred before swallowing. The bread was broken down into smaller particles, and mixed with much larger amounts of saliva. And starch hydrolysis -- or the digestion of starch in the mouth -- was "twice as high for bread as for spaghetti."
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